In order for a shoe to fit properly, it should not only be of the correct length, but also should have width and girth dimensions, particularly in the region where the forepart of the foot is enclosed, that substantially match those of the wearer's foot. As used herein, "girth" is defined as the transverse circumference around the forepart of the foot, typically measured at the critical fitting areas including the ball, waist and instep thereof, and is also used to describe the effective inside circumference of the shoe in the same regions.
Ideally, a customer should be able to purchase a pair of shoes having a separate length and width combination for each foot, reflecting the usual dimensional differences therebetween, and the size selection should be made from a full range of some nine or more widths for each length. Furthermore, the customer should be supplied with means to adjust the width of such shoe, particularly in the critical flexing ball region, to allow further possible girth adjustments during use, to compensate for any stretch in the shoe's upper, and for variations in stocking thickness, and particularly for the usual dynamic changes in the foot itself, which normally varies over a range of up to two full width sizes diurnally, with even greater variations experienced under a variety of specific physiological causal factors affecting mainly accumulation of fluid in the extremities such as is often experienced in warm damp weather, in airline flying, and with a variety of illnesses and/or trauma that may produce similar effects.
In practice, however, the so-called `volume` shoe market, responsible for over 90% of all sales, has found it generally uneconomic to offer shoes in more than one or occasionally two widths, primarily because of the high cost of inventory at the retail level, and the preferred general policy of carrying the widest assortment of styles with the minimum assortment of widths possible. Customers with other than average width feet have a very limited choice, and then usually only at relatively expensive shops, or through mail order operations, both of which are better equipped to handle the inventory problem, although with a considerably more limited choice of styles.
As a result of this situation, it has been accepted that most of all shoes sold provide only an approximate and usually improper fit, particularly in the sensitive and critical ball area where the foot flexes during the stride and where girth adjustment by laces and the like has not proven comfortable or practical.
Attempts to solve this girth adjustment problem by raising the height of the insole, as disclosed in U.S. Pat. No. 3,442,031, have met with limited acceptance, partly since they usually tend to alter the designed tread of the shoe from that of the last on which it was made, "tread" being defined as the relationship of the primarily longitudinal contour of the bottom of the insole in the shoe to the shoe supporting surface, i.e. ground, floor, etc. A further and more profound problem occurs when attempting to correctly fit the usually somewhat differing girths of a person's two feet. Here, the correct fit using this old approach can cause one foot to walk at a higher level than the other, which is orthopedically incorrect and even dangerous not only to the feet, but other portions of the anatomy, including knees, hips, spine and even jaw alignment.
The present invention results from the discovery that the girth of a weight-bearing unshod foot varies significantly with different types of foot supporting surfaces, with the greatest girth produced on a perfectly flat firm surface such as a floor, and the least girth experienced when the same foot is supported by a more conforming surface such as relatively firm but conforming beach sand. This situation holds true for both static and dynamic conditions, with such girth variations found to amount to several consecutive standard width increments. The present invention enables one to duplicate the above conditions faithfully within the shoe itself by its design and construction, and in so doing, to allow the shoe to afford a comfortable and correct fit over a range of several consecutive standard "widths".